The present disclosure relates to a solid-state imaging apparatus and in particular, to a solid-state imaging apparatus installed with a solid-state imaging device such as a CCD (charge coupled device) sensor and a CMOS sensor using a CMOS (complementary metal oxide semiconductor) transistor as a pixel.
FIG. 5 is a schematic cross-sectional view of a solid-state imaging apparatus of the related art disclosed in, for example, JP-A-2002-057311.
For example, a solid-state imaging device 101 such as a CCD sensor and a CMOS sensor is mounted within a recess 100a of a package 100, and necessary wiring is performed by non-illustrated wire bonding or the like.
A color filter 102 is provided on the foregoing solid-state imaging device 101.
Furthermore, a cover glass 103 for protecting the solid-state imaging device 101 and the color filter 102 so as to cover and seal the recess 100a of the package 100.
As described above, a hollow portion 104 is configured by the cover glass 103 and the recess 100a of the package 100, and a solid-state imaging apparatus is mounted within the hollow portion 104 and sealed therein.
In the solid-state imaging apparatus according to the foregoing related art, since radioactive elements such as U (uranium) and Th (thorium) are usually contained in an amount of from 0.1 ppm to 1 ppm in the cover glass 103, there is a problem that the solid-state imaging device 101 is influenced by a crystal defect or the like due to radioactive rays emitted from these elements, especially α-rays 105.
When the α-rays are irradiated on the solid-state imaging device, an energy loss process of α-rays is classified into an electronic energy loss and a nuclear energy loss.
When the energy of α-rays is defined as E, a pair of hole and electron is generated in the number of (n≈E/3.6 eV) in the electronic energy loss.
In the case of a solid-state imaging device, an instantaneous dark current spike is generated from a pixel due to this charge.
Next, though a crystal defect is generated in the nuclear energy loss, this crystal defect generates a defect level in silicon. In the case of a solid-state imaging device, a dark current in a pixel and a transfer part is increased.
With respect to a stored charge-output signal voltage conversion efficiency (η) of the solid-state imaging device, (η=5 to 20 μV/e) is realized. When η=10 μV/e, an image defect (point defect) is confirmed at from approximately 102 e (number of generated electrons) at the time of a field storage (1/60 seconds).
Furthermore, the solid-state imaging device has activation energy of from 0.5 to 0.8 eV due to the energy level of the defect, and the foregoing influence becomes remarkable at the time of high-temperature use.
As the influence of α-rays against, for example, a semiconductor memory, there is known a memory malfunction (software error) caused due to a charge generated by α-rays as described previously.
The software error requires a charge of 106 e (number of generated electrons), and there is a difference of from approximately 103 to 104 times of the number of generated electrons to be confirmed as an image defect (point defect) in a solid-state imaging apparatus.
Since the higher the incident energy of α-rays (from 4 to 9 MeV in a radioactive isotope contained in a material), the more the influence of the software error, a protection structure for decaying the incident energy is effective.
In contrast thereto, in the solid-state imaging device, with respect to its permanent damage, when a depth of a sensor part is approximately 3 μm, the nuclear energy loss becomes the maximum at α-rays of incident energy of approximately 500 keV, the influence of incident α-rays at low energy is very large, and the solid-state imaging device is of an optical input. Accordingly, there are restrictions in the surface protection structure, and a countermeasure to α-rays of the cover glass is important.
In view of the foregoing problems, by realizing a high purity of a raw material of the cover glass 103, thereby decreasing the concentration of U and Th to approximately 30 ppb, it is possible to suppress an influence such as a crystal defect caused due to α-rays. However, the costs of the cover glass become very high.
JP-A-6-021414 is also exemplified as a related art.